A major factor separating Middle English from Modern English is known as the Great Vowel Shift, a radical change in pronunciation during the 15th, 16th and 17th Century, as a result of which long vowel sounds began to be made higher and further forward in the mouth (short vowel sounds were largely unchanged). In fact, the shift probably started very gradually some centuries before 1400, and continued long after 1700 (some subtle changes arguably continue even to this day). Many languages have undergone vowel shifts, but the major changes of the English vowel shift occurred within the relatively short space of a century or two, quite a sudden and dramatic shift in linguistic terms. It was largely during this short period of time that English lost the purer vowel sounds of most European languages, as well as the phonetic pairing between long and short vowel sounds.